There has been known a technique of optically trapping a micro object by irradiating the micro object in a medium with a focused optical vortex (refer to Patent Literature 1 and Non-Patent Literature 1). The optical vortex has a phase singularity on a propagation axis, has a light intensity of zero on the propagation axis, and has a doughnut-shaped light intensity distribution in which the light intensity is at a maximum at a certain distance from the propagation axis.
In addition to the doughnut-shaped light intensity distribution, the optical vortex is also distinctive in that it has an orbital angular momentum. When the micro object in the medium is irradiated with the optical vortex which has the orbital angular momentum, the micro object receives the angular momentum from the optical vortex to revolve around the propagation axis along an orbit having the large light intensity. That is, the optical vortex is capable of optically trapping the micro object in the medium as well as of controlling motion of the micro object.